1. Field of the Invention
The present invention relates to a nonvolatile memory using thin-film transistors that are formed by using the SOI (silicon on insulator) technology and, particularly, to an EEPROM (electrically erasable and programmable read-only memory) that is formed on an insulative substrate so as to be integral with its peripheral circuits such as a driver circuit. The term “silicon” as used above means a silicon single crystal or a silicon semiconductor that is substantially a single crystal.
2. Description of the Related Art
In recent years, the miniaturization of semiconductor devices have required memories that has high performance and large storage capacity and is small in size. Currently, among various storage devices for semiconductor devices, the magnetic disk and the semiconductor nonvolatile memory manufactured by using bulk silicon are used most frequently.
Although the magnetic disk is one of storage devices having the largest storage capacity among those used for semiconductor devices, it has disadvantages of difficulty in miniaturization and slow write and read speeds.
On the other hand, although at present the semiconductor nonvolatile memory is lower in storage capacity than the magnetic disk, its read and write speeds are tens of times higher than the magnetic disk. Further, semiconductor nonvolatile memory products having sufficient performance also in the number of allowable rewrite operations and data holding time have been developed recently. This has caused a tendency of using a semiconductor memory as a replacement of a magnetic disk.
However, conventionally, the semiconductor nonvolatile memory is manufactured by using bulk silicon and accommodated in a package. Therefore, when such a semiconductor nonvolatile memory is mounted on a semiconductor device, the number of manufacturing steps increases and a large-sized package is an obstacle to miniaturization of the semiconductor device.